Field of the Invention
The present invention relates to cleaning baths and to a process for removing a coating containing niobium from a substrate.
Niobium and niobium alloys are known for their advantageous chemical and electrical properties, especially their corrosion resistance towards the majority of corrosive substances at ambient temperature and, above all, their properties of being an excellent conductor of heat and of electricity. As a result of this, niobium and it alloys are frequently used for the construction of chemical engineering plants and equipment and also as a superconductive coating for electrodes. It is known, in fact, that the electrical resistivity of niobium and its alloys becomes virtually zero below 8.3K (Revue de la Societe Royale Belge des Ingenieurs et des industriels, No. 8/9, 1969: R. Winand, "Proprietes et usages des metaux refractaires" ["Properties and uses of refractory metals"], pages 381 to 415). However, the high price of niobium often causes it to be used in the form of a coating on a substrate (R. Lescarts--Dictionnaire des metaux non ferreux [Dictionary of nonferrous metals], 1972, p. 107).
In the construction or maintenance of electrodes or other components bearing a coating containing niobium it may be necessary to dissolve or to clean off the coating in order to release the substrate completely or in part.
In order to accomplish the removal of the niobium, it has been proposed to use, at a temperature below 50.degree. C., a cleaning solution containing 45 to 65 parts by volume of water, 30 to 50 parts by volume of concentrated sulphuric acid, 0.2 to 3.0 parts by volume of a concentrated aqueous solution of hydrogen peroxide and 1.0 to 5.0 parts by volume of concentrated hydrofluoric acid (Central Patents Index, Basic Abstracts Journal, Section L, Week D. 19, Derwent Publications Ltd., London, abstracts 33525D-L: Patent Application JP-A-56-029,324 (Tokyo Shibaura Elec. Ltd)). The cleaning performed in this way is generally quick and efficient, but the presence of hydrofluoric acid makes these known baths toxic and hazardous for users and, consequently, gives rise to polluting wastes. Furthermore, these baths are inadvisable in the case of a copper substrate which is liable to be corroded by them.